Silicon rich nitride CMOS-compatible light sources and Si-based laser structures

ABSTRACT

A fabrication method produces Si compatible light-emitting materials showing sizeable optical gain by thermally annealing thin film layers of Si-rich nitride (SiN x ) By utilizing the Si compatible light-emitting material, light emitting devices can be fabricated that are compatible with CMOS processes. The Si compatible light-emitting material is a high index (refractive index ranging from 1.6 to 2.3) material allowing flexible design of high confinements photonic devices with strong structural stability with respect to annealing treatments. The Si compatible light-emitting material realizes broad band light emission by allowing resonant coupling with rare earth atoms and other infrared emitting quantum dots and better electrical conduction properties with respect to SiO 2  systems. The Si compatible light-emitting material also realizes high transparency (low pumping and modal losses) in the visible range.

PRIORITY INFORMATION

The present patent application claims priority under 35 U.S.C. § 119 from U.S. Provisional Patent Application Ser. No. 60/565,164, filed on Apr. 23, 2004; from U.S. Provisional Patent Application Ser. No. 60/564,900, filed on Apr. 23, 2004; and from U.S. Provisional Patent Application Ser. No. 60/631,041, filed on Nov. 24, 2004. The entire contents of U.S. Provisional Patent Application Ser. No. 60/565,164, filed on Apr. 23, 2004; U.S. Provisional Patent Application Ser. No. 60/564,900, filed on Apr. 23, 2004; and U.S. Provisional Patent Application Ser. No. 60/631,041, filed on Nov. 24, 2004, are hereby incorporated by reference.

GOVERNMENT RIGHTS NOTICE

The present invention was made with government support under Grant (Contract) Number, DMR 02-13282, awarded by the National Science Foundation. The US Government has certain rights to this invention.

FIELD OF THE PRESENT INVENTION

The present invention is directed to light emitting devices or structures having light emitting Si compatible material. More particularly, the present invention is directed to light emitting devices or structures having light emitting Si compatible material that demonstrates optical gain.

BACKGROUND OF THE PRESENT INVENTION

It has been a goal of silicon microphotonics to realize an effective on chip silicon-based light source that allows for both low cost optical functionalities and full VLSI compatibility. However, light emission from bulk silicon is an indirect photon mediated process with low probability. Also, competing non-radiative recombination paths (such as Auger effects or free carrier absorption) severely prevent efficient photon emission and population inversion.

In an effort to engineer materials strategies suitable of efficient light emission from silicon-based structures, conventional approaches have utilized silicon nanocrystals (Si-nc) and rare earth doping of Si-nc to improve room temperature emission efficiency and materials stability. These conventional approaches have almost exclusively relied upon the formation of silicon nanocrystals within SiO₂ matrices, and as such, the conventional approaches are difficult to integrate with the requirements of efficient electrical injection. Moreover, Si/SiO₂ phase separation and subsequent Si-nc nucleation only occur after high temperature annealing treatments in SiO₂ matrices, typically in the range 1100° C.-1250° C., thus preventing full CMOS-VLSI compatibility.

Therefore, it is desirable to develop Si-based materials solutions that can afford efficient room temperature light emission, more efficient electrical injection, more efficient electroluminescence, and improved device stability. Moreover, it is desirable to develop new Si-based material solutions that provide Si-based light amplification with efficient current injection. Furthermore, it is desirable to develop new Si-based material solutions, which can afford intense light emission and/or provide Si-based light amplification with efficient current injection, that are fully CMOS compatible.

SUMMARY OF THE PRESENT INVENTION

One aspect of the present invention is a micro-ring laser that includes a Si/SiN_(x) micro-ring; a Si-based bus waveguide; and a tunable pump laser.

Another aspect of the present invention is a vertical emission Fabry-Perot microcavity laser. The vertical emission Fabry-Perot microcavity laser includes an active laser material and a Si/SiN_(x) Bragg reflector on either side of the active laser material.

A further aspect of the present invention is a vertical emission Fabry-Perot microcavity laser. The vertical emission Fabry-Perot microcavity laser includes an active laser material and a SiO₂/Si₃N₄ Bragg reflector on either side of the active laser material.

Another aspect of the present invention is a waveguide. The waveguide includes a Si substrate and a SiN_(x) light-emitting ridge structure formed upon the Si substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment or embodiments and are not to be construed as limiting the present invention, wherein:

FIG. 1 is a schematic diagram of a micro-ring laser in accordance with the concepts of the present invention;

FIG. 2 is a schematic diagram of a vertical emission Fabry-Perot microcavity laser in accordance with the concepts of the present invention;

FIG. 3 illustrates a Si-rich Si₃N₄ (SiN_(x)) light emitting ridge waveguide in accordance with the concepts of the present invention;

FIG. 4 graphically illustrates the measured relationship between emission and annealing temperatures in accordance with the concepts of the present invention;

FIGS. 5 and 6 graphically illustrate the measured relationship between photoluminescence spectra and observation temperature in accordance with the concepts of the present invention;

FIGS. 7 and 8 graphically illustrate the superlinear increase in the emission intensity after annealing with respect to the optical pump power in accordance with the concepts of the present invention;

FIG. 9 graphically illustrates the measured relationship between SiN_(x) photoluminescence lifetime and pump power in accordance with the concepts of the present invention;

FIGS. 10 and 11 graphically illustrate a typical measured relation of light emission intensity and excitation length; and

FIG. 12 graphically illustrates the measured relationship between material optical gain and pump power in accordance with the concepts of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention will be described in connection with preferred embodiments; however, it will be understood that there is no intent to limit the present invention to the embodiments described herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the present invention as defined by the appended claims.

For a general understanding of the present invention, reference is made to the drawings. In the drawings, like reference numbering has been used throughout to designate identical or equivalent elements. It is also noted that the various drawings illustrating the present invention may not have been drawn to scale and that certain regions may have been purposely drawn disproportionately so that the features and concepts of the present invention could be properly illustrated.

As noted above, the present invention is directed to light emitting devices or structures having light emitting Si compatible material that demonstrates optical gain. More specifically, the present invention is directed to developing new Si-based material solutions that can afford intense light emission. The Si-based material of the present invention provides Si-based light amplification with efficient current injection. Moreover, the Si-based material of the present invention, which can afford intense light emission and/or provide Si-based light amplification with efficient current injection, is fully CMOS compatible.

To fabricate the light emitting Si compatible material of the present invention, a fabrication process utilizes thin film deposition of dielectrics followed by thermal annealing treatments that activates efficient room temperature light emission. In one embodiment of the present invention, the thin film dielectric may be Si-rich nitride (SiN_(x)).

The fabrication process of the present invention includes the deposition of thin SiN_(x) films through plasma enhanced chemical vapor deposition. However, several other thin-films fabrication procedures can be utilized. In one example, silicon rich nitride layers are deposited using SiH₄ and N₂ as precursors and the substrate temperature during deposition is about 400° C. It is noted that crucial to the activation of efficient light emission from the deposited material is the realization of a post-growth annealing process.

Within a fully VLSI-CMOS compatible annealing window, the fabrication process of the present invention produces devices that show efficient room temperature light emission and are characterized by little absorption losses in the visible range. Moreover, the material produced following the deposition and annealing procedure of the present invention shows sizeable optical gain in the spectral region around 1000-1200 nm.

It is noted that the luminescence band can further be tuned by deposition of oxynitride (SiON_(x)) thin films with variable stoichiometry.

Furthermore, it is noted that low temperature pre-annealing processes followed by higher temperature thermal annealing treatments in forming gas atmosphere can be utilized to control the spectral width of the emission band.

In one fabrication embodiment, various annealing treatments, ranging from 400° C. up to 1300° C. enable the fabrication of photonic structures that have a greater degree of flexibility and light emission control than structures produced by conventional fabrication processes. It is noted that the annealing time is determined according to the structure composition wherein the annealing time ranges from 1 minute to several hours.

In one embodiment of the present invention, the annealing process is carried out at a temperature of 800° C. for about ten minutes. As demonstrated in FIG. 4, the annealing temperature of 800° C. achieved the maximum peak for light emission intensity.

It is noted, as demonstrated in FIG. 5, the photoluminescence spectra narrows dramatically as the temperature decreases and in addition, as demonstrated in FIG. 6, the light emission intensity is only weakly dependent upon temperature.

FIGS. 7 and 8 demonstrate super-linear light emission intensity (superradiance) for two light-emitting devices fabricated utilizing the annealing treatment of the present invention. More specifically, FIGS. 7 and 8 demonstrate intense light emission with strong super-luminescence behavior. This super-luminescence behavior suggests the occurrence of amplified spontaneous emission related to stimulated emission in the material. This is suggestion is further supported by FIG. 9 which demonstrates emission lifetime shortening which results from stimulated emission in the material.

To measure optical gain in a structure fabricated using the procedures of the present invention, a standard variable stripe length technique under continuous wave optical pumping can be utilized. In utilizing this measurement technique, devices fabricated with Si-rich, Si₃N₄, and oxynitride (SiON_(x)) films and utilizing the annealing treatment of the present invention demonstrated that at low pumping rates, only optical losses can be observed, and at higher pumping rates, the losses switch into net optical modal gain. The gain results as a function of the pumping conditions are demonstrated in FIGS. 10-12. It is noted that FIGS. 7-11 demonstrate the presence of optical gain in the proposed material and motivate its use a viable approach for a Si-base light amplifier.

The emission and optical gain mechanism is most likely related to the presence of nitrogen luminescence centers in small silicon clusters that nucleate after the thermal annealing process, as described above.

The presence of nitrogen related luminescence centers in silicon clusters material can be utilized in Si-based on-chip optical amplifiers, light emitting waveguide structures, compact micro-ring laser devices, and due to its high refractive index, light emitting photonic crystal structures.

It is noted that the utilization of SiN_(x) as a high refractive index and a broad band light emitting material enables effective transfer of the excitation to rare earth atoms (for instance erbium) through energy coupling mechanisms. The emission relies on the formation of nitrogen passivated silicon clusters dispersed in the embedding Si₃N₄ dielectric host, in close analogy with Si/SiO₂ superlattice systems. In other words, the inclusion of rare earth doping within the nitride or oxynitride structures fabricated by the processes of the present invention produces light emitting photonic structures characterized by efficient near infrared emission with an improved degree of material stability.

As noted above, the fabrication process of the present invention can be utilized to realize different light emitting photonic structures schemes.

FIG. 1 illustrates an example of photonic device fabricated utilizing Si-rich, Si₃N₄, and oxynitride (SiON_(x)) films and the annealing treatment of the present invention. As illustrated in FIG. 1, a micro-ring laser includes a Si/SiN_(x) rich micro-ring 10, a Si-based bus waveguide 20, and a tunable pump laser 30. Due to the high refractive index of the Si-rich nitride films of micro-ring 10, small curvature radii can be realized. Small curvature radii enable an ultra-compact planar ring design; preferably, the dimensions are between 3 and 5 μm. A Si/SiN_(x) rich micro-ring fabricated utilizing Si-rich Si₃N₄ and/or oxynitride (SiON_(x)) films and the annealing treatment of the present invention micro-ring laser capable of pump, preferably 488 nm, and signal, preferably 1150 nm, trapping. The annealing treatment of the present invention, preferably at temperatures ranging between 400° C. and 1300° C. in N₂ atmosphere, activates the ring photoluminescence and provides smoothing of the ring wall interface's roughness, thereby allowing a better quality factor.

It is noted that that Si-rich silicon nitride (SiN_(x)) based micro-ring laser fabricated using the concepts of the present invention, namely the described processes to fabricate the light emitting material and to activate efficient light emission, can accomplish both light trapping in the ring and efficient room temperature light emission.

FIG. 2 illustrates a vertical emission Fabry-Perot microcavity laser that includes transparent Si/SiN_(x) or SiO₂/Si₃N₄ Bragg reflectors 40 and an active laser material 50. The transparent Si/SiN_(x) or SiO₂/Si₃N₄ Bragg reflectors 40 are fabricated on either side of the active laser material 50. The active laser material 50 may be SiN_(x).

Another example, of photonic structure fabricated by the process of the present invention is illustrated in FIG. 3. As illustrated in FIG. 3, a waveguide includes lateral contacts 300, an active region 200, and a substrate 100. The active region 200 is preferably SiN_(x).

In the device illustrated by FIG. 3, a Si-rich Si₃N₄ light emitting ridge waveguide was fabricated upon a silicon substrate. In such a device, a SiO₂ under-cladding layer is utilized. This Si-rich Si₃N₄ light emitting ridge waveguide with a SiO₂ under-cladding layer provides better modal confinement.

In other words, the waveguide includes a Si substrate, a SiN_(x) light-emitting ridge structure formed upon the Si substrate, and a SiO₂ under-cladding layer. The SiN_(x) light-emitting ridge structure is constructed of thermally annealed thin film layers of SiO_(x) and SiN_(x).

Upon utilizing the annealing treatment of the present invention, the waveguide structure of FIG. 3, can be activated in order to produce strong light emission and optical amplification.

It is noted that the waveguide structure, as described above with respect to FIG. 3, also represents SiN_(x) integrated light amplifiers in a planar waveguide geometry. The SiN_(x) integrated light amplifiers in a planar waveguide geometry allow for large light interaction length with the active medium and are suitable for optical and electrical pumping schemes if the lateral electrodes are fabricated on either sides of the ridge. Such a waveguide structure can be pumped optically by an off-chip tunable laser source in a top or co-propagating pumping geometry. In addition, electrical pumping can be achieved.

In summary, the fabrication process of the present invention is entirely compatible with CMOS processes; utilizes high index (refractive index ranging from 1.6 to 2.3) material to allow flexible design of high confinements photonic devices with strong structural stability with respect to annealing treatments; realizes broad band light emission by allowing resonant coupling with rare earth atoms and other infrared emitting quantum dots; realizes better electrical conduction properties with respect to SiO₂ systems; and/or enables high transparency (low pumping and modal losses) in the visible range.

While the present invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 

1. A micro-ring laser, comprising: a Si/SiN_(x) micro-ring; a Si-based bus waveguide; and a tunable pump laser.
 2. The micro-ring laser as claimed in claim 1, wherein said Si/SiN_(x) micro-ring is constructed of thermally annealed thin film layers of SiO₂ and SiN_(x).
 3. A vertical emission Fabry-Perot microcavity laser, comprising: an active laser material; and a Si/SiN_(x) Bragg reflector on either side of said active laser material.
 4. The vertical emission Fabry-Perot microcavity laser as claimed in claim 3, wherein each Si/SiN_(x) Bragg reflector is constructed of thermally annealed thin film layers of Si-rich oxide (SiO_(x)) and Si-rich nitride SiN_(x).
 5. A vertical emission Fabry-Perot microcavity laser, comprising: an active laser material; and a SiO₂/Si₃N₄ Bragg reflector on either side of said active laser material.
 6. The vertical emission Fabry-Perot microcavity laser as claimed in claim 5, wherein each SiO₂/Si₃N₄ Bragg reflector is constructed of thermally annealed thin film layers of Si-rich oxide (SiO_(x)) and Si-rich Si₃N₄.
 7. A waveguide comprising: a Si substrate; and a SiN_(x) light-emitting ridge structure formed upon said Si substrate.
 8. The waveguide as claimed in claim 7, wherein said SiN_(x) light-emitting ridge structure is constructed of thermally annealed thin film layers of SiO_(x) and SiN_(x).
 9. The waveguide as claimed in claim 8, further comprising a SiO₂ under-cladding layer. 